This invention can be used with the invention described in application Ser. No. 871,444 filed concurrently herewith by Gunter Gleim, Friedrich Fuldner and Bernd Rekla and titled "Detector Circuit", which application is a continuation of PCT application PCT/EP 90/01595, filed Sep. 20, 1990.
This invention can be used with the invention described in application Ser. No. 871,445 filed concurrently herewith by Gunter Gleim, Friedrich Fuldner and Bernd Rekla and titled "Direction-Determination Logic", which application is a continuation of PCT application PCT/EP 90/01594, filed Sep. 20, 1990.
The invention is directed to a method of determining the direction of travel of a scanner across markings or data tracks on a recording medium in which a first error signal and a second error signal shifted in phase from the first error signal is generated. CD players, video disc players, DRAW disc players or magneto-optical recording and playback apparatus, for example, are equipped with a track regulation circuit and an optical scanning device.
The construction and function of an optical scanning device, a so-called optical pick-up, are described in Electronic Components & Applications, Vol. 6, No. 4, 1984, on pages 209 through 215. Lenses focus a light beam emitted from a laser diode onto a compact disk, which reflects it onto a photodetector. The information stored on the disk, and the actual values for the focusing and tracking circuits are obtained from the signal reflected from the detector. The referenced article calls the deviation of the focusing-circuit actual value from its reference value the focusing error, and the deviation of the tracking-circuit actual value from its reference value the radial tracking error.
the focusing circuit is adjusted with a coil having an object lens which moves along an optical axis through the magnetic field of the coil. The focusing circuit moves the lens back and forth to maintain the light beam from the laser diode focused on the compact disk. The tracking circuit, which is often called the radial drive mechanism, moves the optical pickup radially over the disc.. The radial-drive mechanism in some equipment includes a coarse-driven mechanism and a fine-drive mechanism. The coarse-drive mechanism can include a spindle that shifts the overall optical pickup, consisting of a laser diode, lenses, a prismatic beam splitter, and a photodetector, back and forth radially. The fine-drive mechanism can either shift the beam of light back and forth radially or tilt it at a prescribed angle, advancing it slightly, approximately 1 mm, along a radius of the disc.
High quality reproduction, irrespective of whether the data are both picture and sound in a videodisc player, sound alone in a compact-disc player, or the data stored on a magneto-optical disc, requires precise focusing of the light beam onto the disc and also precise guidance along the data tracks of the disc.
FIGS. 1 to 4 are useful in understanding how the track of a laser scanned disc is followed. In FIG. 1, three laser beams L1, L2, and L3 are focused onto a photodetector PD in the optical pickup of a compact-disk player, the direction of motion of detector PO relative to the disc is indicated by the arrow. Beams L2 and L3 are diffraction beams of orders +1 and -1. A pickup of this type is called a three-beam pickup. The photodetector PD includes four square photodiodes A, B, C and D arrayed in the form of a larger square. A rectangular photodiode E is arranged in front of the diodes A to D and another photodiode F is arranged behind the photodiodes A to D. The middle laser beam, beam 1, is focused onto photodiodes A, B, C, and D, to generate data signal HF=AS+BS+CS+DS and a focusing-error signal FE=(AS+CS)-(BS+DS). The forward outer beam L3 is reflected to photodiode E and rear outer beam L2, is reflected to photodiode F. The two outer beams L2 and L3 provide signals for the generation of a tracking-error signal TE=ES-FS. The parameters AS, BS, CS, DS, ES, and FS are the photoelectric voltages provided by the photodiodes A, B, C, D, E, and F, respectively.
When the middle laser beam L1 is precisely at the middle of a track the tracking-error signal TE has the value zero: EQU TE=ES-FS.
When the middle beam moves away from the middle of the track S, one of the diffraction beams approaches the middle of the track and the other diffraction beam shines on the space between two tracks. Since, however, a track reflects differently from the space between the two tracks, one diffraction beam will be reflected more powerfully than the other.
Laser beams L1, L2, and L3 are displaced to the right of track S in FIG. 2, and the tracking-error signal assumes a negative value:ps EQU TE=ES-FS&lt;0.
The mechanism that adjusts the tracking circuit shifts the optical pickup to the left until tracking-error signal TE becomes zero.
In the opposite situation, when the laser beams have been displaced to the left of the track, the tracking-error signal becomes positive: EQU TE=ES-FS&lt;0.
The mechanism that adjusts the tracking circuit shifts the optical pickup to the right until the tracking-error signal becomes zero. This situation is illustrated in FIG. 3.
When middle beam L1 and its associated diffraction beams L2 and L3 cross several data tracks, tracking-error signal TE assumes the sinusoidal shape illustrated in FIG. 4.
A tracking circuit is described in Japanese Exposure 60 10429. In this tracking circuit, the lower and upper envelope of the HF signal indicates whether a beam of light is crossing any data tracks. When the beam travels over several tracks, the HF signal collapses uniformly between two tracks. The number of tracks crossed by the beam of light is determined by constructing, the envelope of the HF signal and converting the envelope into a square-wave signal that is supplied to the counting input terminal of an up-and-down counting circuit, which counts the HF breakdowns.
Direction determining logic is used to determine the direction of the radial motion of the light beam across the recording medium. This logic evaluates the phase shift between the tracking error signal TE and the envelope of the HF signal.
Patent GB-A 2 073 543 describes a tracking regulation circuit which checks whether the HF signal lies below a predetermined threshold value. When it does, dependent upon the sign of the preceding tracking error signal, either a positive or a negative voltage is applied to the control unit of the optical scanner to guide the scanner onto the right data track. However, because dust, dirt, fingerprints or scratches on a CD disk can also cause a collapse of the HF signal, measures must be taken to distinguish between HF collapses caused by such conditions of the recording medium from HF collapses caused by tracking changes of the light beam.
Patent EP-A 0 183 303 describes a CD player in which the lock-in of the light spot onto a data track of the compact disk, the so-called locking-in, occurs at the point of the largest eccentricity of the disk because at this point the relative speed between the light beam and the data track is the lowest. In order to determine the number of tracks crossed by the light beam the envelope of the HF signal is generated and compared with a threshold value. The comparison of the envelope of the HF signal with the first threshold value provides a pulse shaped signal. Each pulse of this pulse shaped signal indicates a change of track. In order to render the CD player more secure against so-called drop-outs--i.e. audible interference in sound reproduction due to a defective, scratched or dirty compact disk--the envelope of the HF signal can be compared with a second threshold value. A pulse is generated from the envelope of the HF signal only when the envelope exceeds both the first and the second threshold value and when the drop-out detector of the CD player is not in operation.
Patent WO-A-88/09988, describes a CD player with a drop-out detector which evaluates the HF signal. This tracking regulation circuit is disadvantageous in that upon a reversal in the direction of the light beam the phases of the HF signal which generates the counting pulses and the tracking error signal must be taken into consideration because errors can be caused by improper phasing.